1. Field of the Invention
The present invention relates to an oscillation drive device including a voice coil motor that makes an oscillatory movement thereby shifting pixels of an image projected by a projection-type image display apparatus.
2. Description of the Related Art
Recently, a pixel shifting device has been employed in a projection-type image display apparatus that projects a light on an image thereby projecting the image onto a screen. The pixel shifting device includes a glass through which the projected light passes before reaching the screen and an actuator (an oscillation drive device) that causes the glass to oscillate. Because of the oscillation of the glass, the projected light oscillates and slightly shifts before reaching the screen. The glass is oscillated in such as manner that each pixel of the image is shifted by a half pixel on the screen. By such a pixel shift, the number of apparent pixels is increased, so that the resolution of the image can be improved.
A voice coil motor is typically used as the actuator. The voice coil motor includes a cylindrical coil, a cup-like outer yoke that encloses the coil from the outside, a permanent magnet that is arranged in the center of inside the outer yoke (inside the coil), and an inner yoke that is arranged on an upper end of the permanent magnet.
On the other hand, oscillatory apparatuses are known in the art. Such oscillatory apparatuses also employ a cylindrical voice coil motor to produce oscillation. In case of the oscillatory apparatuses, however, the center of oscillation is located outside the voice coil motor and also located midway between a moving part of the oscillatory apparatus and the voice coil motor. Such an oscillatory apparatus has been disclosed, for example, in Japanese Patent Application Laid-open No. H5-326603 (see page 2 and FIG. 1).
A cylindrical voice coil motor efficiently produces a thrust force over the entire circumference of a coil. However, a cup-like yoke and the like used in a cylindrical voice coil motor is difficult to machine. Therefore, there is generally employed such a voice coil motor that a yoke (a center yoke) made of a metal plate is arranged in a hole portion of a rectangular toroidal coil, and one magnet is arranged on a position across one side of the coil.
In this voice coil motor, it is configured that an end of a yoke arranged on the back side of the magnet (a back yoke) is in contact with an end of the center yoke. By this configuration, a magnetic flux of the magnet runs in cycle, and thereby improving the efficiency of the voice coil motor.
In the voice coil motor including the rectangular toroidal coil, if a side of the coil located farthest away from the center of oscillation is caused to function as the voice coil motor, it is possible to increase the torque. Furthermore, if the side of the coil serving as the voice coil motor is arranged farther away from the center of oscillation, it is possible to obtain a higher torque with a lower thrust force.
However, typical coils are made of copper wire, so that the coil is relatively large in mass among oscillatory components. Therefore, if the side of the coil serving as the voice coil motor is arranged too far away from the center of oscillation, the moment of inertia is increased, and thus it is difficult to achieve a high-speed oscillatory movement. Furthermore, an oscillation drive device for shifting pixels has been expected to be downsized in accordance with the development of a slimmed-down and downsized projection-type image display apparatus.